A Computed Tomography system may collect data by using a large flat panel digital x-ray device, imager, or detector, having a plurality of pixels arranged in rows and columns. Large flat panel imagers function by accumulating charge on capacitors generated by the pixels of photodiodes (amorphous silicon or organic semiconductor) with scintillators or by pixels of photoconductors. Typically, many pixels are arranged over a surface of the imager where TFTs (or single and/or double diodes) at each pixel connect the charged capacitor to a read-out amplifier at the appropriate time. A pixel is composed of the scintillator/photodiode/capacitor/TFT or switching-diode combination or by the photoconductor/capacitor/TFT or switching-diode combination. Often the photodiode intrinsically has enough capacitance that no separate charge storage capacitor is required. Radiation (e.g., alpha, beta, gamma, X-ray, neutrons, protons, heavy ions, etc.) strikes the scintillator and causes the scintillator to generate visible light. The visible light strikes a photodiode and generates an electric current. Alternatively, an imager may be configured such that the radiation strikes a biased photoconductor to generate the electric current. The current charges a capacitor and leaves a charge on the capacitor. The integrated charge on the capacitor is proportional to the integrated light intensity striking the respective photoconductor for a given integration time. At an appropriate time, a switch (e.g., a TFT or switching diode(s)) activates and reads out the charge from the capacitor.
However, such flat panel imagers suffer from detector lag. The detector lag causes a significant portion of the signals from previous samples to incorrectly bias subsequent samples. A significant cause of the lag is related to the electron de-trapping resulting from the high density electronic defects in the energy band gap. De-trapping times range from a few milliseconds to as long as 100 seconds, days, weeks, or even months. As a result of the non-uniformity of the lag, artifacts, such as rings and bands, occur in the reconstructed images.
Prior correction methods have been implemented to estimate offset correction. One prior background correction method discussed in U.S. Pat. No. 5,249,123 applies a filter function to the output signal. Another prior background correct method discussed in U.S. Pat. No. 6,701,000 energizes each detector pixel to reduce residual signal in-between data acquisitions when the x-ray emission is stopped. In Overdick, a hardware solution is described to eliminate the gain and lag effects from flat panel detectors. LEDs that are built into the panel saturate the traps in the detector, which minimize the temporal artifacts.
Some of the proposed methods have assumed that the lag was the result of a linear time invariant process. However, this may be false because the time constants associated with traps filling with charge and releasing charge can be different. Thus, a correction based on assuming linearity and time invariance will not fully correct the data. The solution presented in Overdick requires a hardware modification to existing flat-panel x-ray detectors that do not already have the LED electronics built-in.